Method for treating hexavalent chromium-containing aqueous solution

ABSTRACT

Provided is a method for treating a hexavalent chromium-containing aqueous solution by water treatment employing a titanium dioxide photocatalyst that is excellent in both photocatalytic activity and solid-liquid separation performance. The method according to the present disclosure includes the steps of: adding catalyst particles to the aqueous solution; reducing hexavalent chromium by irradiating the aqueous solution with light having a wavelength of 200 nanometers or more and 400 nanometers or less while stirring the catalyst particles in the aqueous solution; and stopping the stirring and separating the catalyst particles from the aqueous solution by sedimentation. Each catalyst particle is composed only of a titanium dioxide particle and a zeolite particle, the titanium dioxide particle is adsorbed on the outer surface of the zeolite particle, the zeolite particle has a silica/alumina molar ratio of 10 or more, and the catalyst particles are contained in the aqueous solution at a concentration of 0.4 grams/liter or more and 16 grams/liter or less.

This is a continuation of International Application No.PCT/JP2013/003590, with an international filing date of Jun. 6, 2013,which claims the foreign priority of Japanese Patent Application No.2012-134414, filed on Jun. 14, 2012, the entire contents of both ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a method for treating a hexavalentchromium-containing aqueous solution.

2. Description of Related Art

In resent years, the use of a photocatalyst has been expected as meansfor treating water containing a predetermined pollutant. For example,Yuko Maruo and two other persons state that a medical drug contained inwater can be decomposed and removed by a titanium dioxide photocatalyst(“Development of dispersion-type TiO₂ photocatalyst for decomposition ofmedical drugs in water”, Book of Preprints of The 77th annual meeting ofThe Society of Chemical Engineers, Japan, Public Interest IncorporatedAssociation The Society of Chemical Engineers, Japan, March 2012, p.427). In addition, Limin Wang and two other persons state thathexavalent chromium can be reduced into trivalent chromium byphotocatalytic reaction of titanium dioxide, and that the presence ofcoexisting organic substances and the surface area of the titaniumdioxide influence the reduction rate (“Photocatalytic reduction ofCr(VI) over different TiO₂ photocatalysts and the effects of dissolvedorganic species”, Journal of Hazardous Materials, Mar. 21, 2008, vol.152, No. 1, p. 93-99). In addition, in order to facilitate solid-liquidseparation of photocatalyst particles dispersed in water, it has beenproposed to use a photocatalyst in which titanium dioxide particles areimmobilized by a binder such as a binding agent on support particleshaving a larger particle diameter than the titanium dioxide particles(see JP 10-249210 A, for example). In addition, a technique has beenproposed that uses a photocatalyst obtained by coating support particleswith titanium dioxide by a coating process such as a sol-gel process(see JP 11-500660 T, for example).

SUMMARY OF THE INVENTION

Although the techniques proposed in JP 10-249210 A and JP 11-500660 Tare suitable for solid-liquid separation of photocatalyst particlesdispersed in water, the techniques may not provide sufficientphotocatalytic activity.

In view of the above findings, one non-limiting and exemplary embodimentprovides a method for treating a hexavalent chromium-containing aqueoussolution by water treatment employing a titanium dioxide photocatalystthat is excellent in both photocatalytic activity and solid-liquidseparation performance.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and Figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

In one general aspect, the techniques disclosed here feature a methodfor treating a hexavalent chromium-containing aqueous solution, themethod including: a step a of adding catalyst particles to the aqueoussolution; a step b of reducing hexavalent chromium by irradiating theaqueous solution with light having a wavelength of 200 nanometers ormore and 400 nanometers or less while stirring the catalyst particles inthe aqueous solution; and a step c of stopping the stirring in the stepb and separating the catalyst particles from the aqueous solution bysedimentation. Each catalyst particle is composed only of a titaniumdioxide particle and a zeolite particle, the titanium dioxide particleis adsorbed on an outer surface of the zeolite particle, the zeoliteparticle includes silica and alumina at a silica/alumina molar ratio of10 or more, and the catalyst particles are contained in the aqueoussolution at a concentration of 0.4 grams/liter or more and 16grams/liter or less.

According to the above method, it is possible to provide a method fortreating a hexavalent chromium-containing aqueous solution by watertreatment employing a titanium dioxide photocatalyst that is excellentin both photocatalytic activity and solid-liquid separation performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram conceptually showing the structure of a titaniumdioxide composite catalyst, FIG. 1B is a diagram conceptually showingthe structure of a catalyst obtained by a binder process, and FIG. 1C isa diagram conceptually showing the structure of a catalyst obtained by asol-gel process.

FIG. 2 is a diagram conceptually showing the configuration of a watertreatment system of a first embodiment.

FIG. 3 is a diagram showing results of experiments for evaluating thenatural sedimentation velocity of particles.

FIG. 4 is a diagram conceptually showing the configuration of a watertreatment system of a second embodiment.

FIG. 5 is a perspective view schematically showing the structure of afiltration membrane element.

FIG. 6 shows graphs representing the particle size distribution oftitanium dioxide particles and the particle size distribution ofcatalyst particles of an embodiment of the present disclosure.

FIG. 7A is a photograph of a transmission electron microscope image oftitanium dioxide particles alone, FIG. 7B is a photograph of atransmission electron microscope image of zeolite particles alone, andFIG. 7C is a photograph of a transmission electron microscope image of atitanium dioxide composite catalyst of an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

<Findings on which the Present Disclosure is Based>

Nowadays, pollution of drinking water or surface water by hexavalentchromium has been reported in various parts of the worlds. Hexavalentchromium is a substance that is extremely toxic for human bodies. Thelethal dose of potassium dichromate, which is a representativehexavalent chromium compound, is 0.5 to 1 gram. In addition, sincehexavalent chromium has carcinogenicity, mutagenicity etc., drinkinghexavalent chromium-containing water continuously over a long period oftime causes cancers etc. In view of such high hazardousness ofhexavalent chromium, many countries strictly regulate the concentrationof hexavalent chromium in drinking water and discharged water. Forexample, in Japan, it is stipulated that the standard value forhexavalent chromium in drinking water is 50 ppb or less and the standardvalue for hexavalent chromium in discharged water is 500 ppb or less.

However, hexavalent chromium is an indispensable substance for industry,and is used in various industrial fields such as plating industry, steelindustry, and tannage industry.

In the natural environment, chromium is present in an oxidized trivalentor hexavalent form. In contrast to hexavalent chromium which is highlytoxic, trivalent chromium has not been found to have toxicity, and is anessential trace metal element for human bodies. Also, trivalent chromiumis exempt from the regulations for the concentration in discharged waterand drinking water. Therefore, hexavalent chromium can be detoxified bybeing reduced into trivalent chromium.

As methods for reducing hexavalent chromium, methods usingphotocatalysts have been reported as well as methods using a chemicalreagent serving as a reducing agent. Among them, a method using atitanium dioxide photocatalyst is expected as a sustainable treatmentmethod since the method does not need an agent such as a chemicalreagent and can use sunlight. The reduction of hexavalent chromium by atitanium dioxide photocatalyst is due to excited electrons generated byphotocatalytic reaction of titanium dioxide. When titanium dioxide isirradiated with ultraviolet light, holes and electrons are generated.The holes react with water molecules to generate OH radicals, and theelectrons generated simultaneously with the holes react with hexavalentchromium adsorbed on the surface of the titanium dioxide. As a result,the hexavalent chromium can be reduced into trivalent chromium.

As the types of the titanium dioxide photocatalyst supplied into aphotoreactor in the water treatment method using the titanium dioxidephotocatalyst, there can be mentioned: (I) an immobilized catalyst inthe case of which nanometer-order titanium dioxide particles are used bybeing immobilized on a substrate with a binder or the like; and (II) adispersed catalyst in the case of which nanometer-order titanium dioxideparticles are used by being mixed with and suspended in water to betreated. In either case, the titanium dioxide photocatalyst isirradiated with UV (Ultraviolet) light for excitation of titaniumdioxide in a state where an interface is formed between the water andthe titanium dioxide photocatalyst. Of the two types of catalysts, thedispersed catalyst denoted by (II) is much more advantageous from thestandpoint of efficiency of reduction of hexavalent chromium since thedispersed catalyst allows a larger surface area per unit mass to beobtained, and also allows a chemical substance to be diffused withoutany disturbance and to reach the surface of the titanium dioxide. Infact, when the performances of (I) and (II) are compared in terms ofefficiency of reduction of hexavalent chromium in water, the dispersedcatalyst denoted by (II) exhibits performance that is ten to one hundredtimes higher than that of the immobilized catalyst denoted by (I).

However, in a water treatment method using a dispersed catalyst,titanium dioxide particles are in a state of being dispersed in waterafter chromium in water is reduced by irradiation with UV light. If thetitanium dioxide particles dispersed in water is separated from thetreated water by solid-liquid separation, reuse of the titanium dioxideparticles and discharge of the treated water are enabled. However, thetitanium dioxide particles have a nanometer-order particle diameter, andtherefore, the solid-liquid separation of the titanium dioxide particlesdispersed in water is difficult. For example, when a separation meansusing a polymer filter is employed, clogging of the filter is caused bythe titanium dioxide particles, and the flow rate of the treated wateris thus decreased, which makes it difficult to perform continuoussolid-liquid separation of the titanium dioxide particles. In addition,in the case of employing a natural sedimentation process using gravity,the sedimentation velocity of the titanium dioxide particles isextremely low due to the very small particle diameter of the titaniumdioxide particles, and therefore, the solid-liquid separation of thetitanium dioxide particles are not completed even when the treated waterin which the titanium dioxide particle are dispersed is allowed to standfor 1 to 2 days. That is, despite its excellent performance in reductionof hexavalent chromium in water, the water treatment method using adispersed catalyst has not been fully put into practical use since thestep of the solid-liquid separation of the titanium dioxide particlesacts as a rate-limiting step in the whole water treatment, andsignificantly hinders the efficiency of the water treatment.

When titanium dioxide particles having an over-nanometer diameter, forexample, a diameter larger than 1 μm, are used as a catalyst,solid-liquid separation by sedimentation is enabled. However, titaniumdioxide particles having a large particle diameter are smaller insurface area per unit mass than nanometer-order titanium dioxideparticles. Moreover, when the diameter of the titanium dioxide particlesis increased, the titanium dioxide makes a phase transition from ananatase crystal which has high photocatalytic activity to a rutilecrystal which has low photocatalytic activity, with the result thatsufficient photocatalytic activity is not obtained. For example, thetechniques described in JP 10-249210 A and JP 11-500660 T have beenproposed in order to realize a titanium dioxide particle photocatalystthat allows solid-liquid separation of titanium dioxide particlesdispersed in water.

When titanium dioxide particles serving as a photocatalyst areimmobilized by a binder on support particles having a larger particlediameter than the titanium dioxide particles, the titanium dioxideparticles are firmly immobilized on the surfaces of the supportparticles. As a result, micrometer-order photocatalyst particlessuitable for solid-liquid separation from water can be obtained.However, the immobilization by this method may decrease thephotocatalytic activity of the titanium dioxide particles. In addition,a photocatalyst including support particles and titanium dioxidedeposited on the support particles by a sol-gel process is indeedsuitable for solid-liquid separation of the photocatalyst particlesdispersed in water, but lacks sufficient photocatalytic activity,similarly to the photocatalyst in which titanium dioxide particles areimmobilized by a binder on support particles having a larger particlediameter than the titanium dioxide particles.

<Description of Aspects of the Present Disclosure>

A first aspect of the present disclosure provides a method for treatinga hexavalent chromium-containing aqueous solution, the method including:a step a of adding catalyst particles to the aqueous solution; a step bof reducing hexavalent chromium by irradiating the aqueous solution withlight having a wavelength of 200 nanometers or more and 400 nanometersor less while stirring the catalyst particles in the aqueous solution;and a step c of stopping the stirring in the step b and separating thecatalyst particles from the aqueous solution by sedimentation. Eachcatalyst particle is composed only of a titanium dioxide particle and azeolite particle, the titanium dioxide particle is adsorbed on an outersurface of the zeolite particle, the zeolite particle includes silicaand alumina at a silica/alumina molar ratio of 10 or more, and thecatalyst particles are contained in the aqueous solution at aconcentration of 0.4 grams/liter or more and 16 grams/liter or less.

According to the first aspect, each catalyst particle is composed onlyof titanium dioxide and a zeolite particle, and the titanium dioxideparticle is adsorbed on the outer face of the zeolite particle.Therefore, almost the whole of a surface active site of the titaniumdioxide particle can be effectively used. In addition, the terminalvelocity of natural sedimentation of the catalyst particle is higherthan that of a single zeolite particle or a single titanium dioxideparticle, and thus excellent solid-liquid separation performance isexhibited.

A second aspect of the present disclosure provides the method as setforth in the first aspect, the method including a step d of adding againthe catalyst particles separated by sedimentation in the step c to theaqueous solution after the step c, wherein the step b and the step c areperformed again after the step d.

According to the second aspect, the separated catalyst particles can bereused.

A third aspect of the present disclosure provides the method as setforth in the first aspect, wherein the catalyst particles are separatedby sedimentation in a solid-liquid separation vessel including afiltration membrane element in the step c, the method further includes astep e of producing treated water from the aqueous solution using thefiltration membrane element, and the filtration membrane element used inthe step e is composed of a plate-shaped frame and sheets of filterpaper made of resin and attached to both faces of the frame, and isarranged parallel to a direction in which the catalyst particles aresedimented.

According to the third aspect, treated water can be produced using thefiltration membrane element.

A fourth aspect of the present disclosure provides the method as setforth in the third aspect, the method including a step f of adding againthe catalyst particles separated by sedimentation in the step c to theaqueous solution after the step c, wherein the step b, the step c andthe step e are performed again after the step f.

According to the above aspect, the catalyst particles separatedaccording to the third aspect can be reused.

A fifth aspect of the present disclosure provides the method as setforth in any one of the first to fourth aspects, wherein the zeoliteparticle is a zeolite particle subjected to a process in which analumina portion is dissolved by treatment with an acid aqueous solutionto introduce an active site for direct adsorption of the titaniumdioxide particle, and then the acid aqueous solution adhered to thesurface of the zeolite particle is removed by washing with water.

According to the fifth aspect, the alumina portion (Al portion) of thezeolite is dissolved, and an increased number of active sites for directadsorption of titanium dioxide can be introduced into the basic skeletonof the zeolite.

Embodiments of the present disclosure will be described below withreference to the drawings. However, the present disclosure is notlimited to the embodiments described below.

<First Embodiment>

FIG. 1A conceptually shows the structure of a titanium dioxide compositecatalyst used in a method of the present embodiment. The titaniumdioxide composite catalyst of the present embodiment includes zeoliteparticles 101 having a micrometer-order particle diameter, and has astructure in which nanometer-order anatase-type titanium dioxideparticles 102 are immobilized on the surfaces of the zeolite particles101. The titanium dioxide composite catalyst is dispersed in water to betreated 107. That is, the titanium dioxide composite catalyst used inthe method of the present embodiment is composed only of titaniumdioxide particles and zeolite particles. FIG. 1B conceptually shows thestructure of a catalyst in the case of which titanium dioxide particlesare immobilized on a support particle 104 by a binder process which is aconventional technique, and FIG. 1C conceptually shows the structure ofa catalyst in the case of which titanium dioxide is immobilized on thesupport particle 104 by a sol-gel process. In the case where thetitanium dioxide particles 102 are immobilized on the support particle104 using a thin film 103 formed of a binder agent as in FIG. 1B, a partof the surface active site specific to the titanium dioxide particle iscovered with a thin film formed of SiO₂, Al₂O₃ or the like derived froma precursor substance. As a result, the titanium dioxide particles areinactivated, and their catalytic performance is deteriorated. Inaddition, in the case where support particles are coated with titaniumdioxide by a sol-gel process, the catalyst has a structure in which, asshown in FIG. 1C, the surface of the support particle 104 is coveredwith a TiO₂ film 105 and a titanium dioxide deposit 106 is present on apart of the outer periphery of the TiO₂. Therefore, the substantialreactive surface area of the titanium dioxide serving as a catalyst issmall. In the titanium dioxide composite catalyst used in the method ofthe present embodiment, as shown in FIG. 1A, the titanium dioxideparticles 102 are immobilized directly on the zeolite particle 101without the mediation of a thin film. Therefore, in the titanium dioxidecomposite catalyst used in the method of the present embodiment, almostthe whole of the surface active site of the titanium dioxide particlecan be effectively used, and photocatalytic activity comparable to thatof a nanometer-order titanium dioxide particle can be ensured.Consequently, the photocatalytic activity of the titanium dioxidecomposite catalyst used in the method of the present embodiment is about8 times higher than that of a photocatalyst prepared by the binderprocess or the sol-gel process. Here, the description “the titaniumdioxide composite catalyst is composed only of titanium dioxideparticles and zeolite particles” means that, as shown in FIG. 1A, thethin film 103 formed of a binder agent is not present on the outersurfaces of the zeolite particle 101 and the titanium dioxide 102, andthe surface of the zeolite particle 101 is not covered with the TiO₂thin film 105.

For example, zeolite particles and titanium dioxide particles are mixedat a predetermined weight ratio in pure water or nearly-pure water, themixed liquid is then immediately subjected to ultrasonic dispersiontreatment to allow the titanium dioxide particles to be adsorbed on thesurfaces of the zeolite particles, and thus the titanium dioxideparticles are immobilized directly on the surfaces of the zeoliteparticles. The purpose of the ultrasonic treatment is to forciblydisperse the titanium dioxide particles intrinsically formingaggregations each consisting of several hundred particles in water, andthereby to facilitate the immobilization of the titanium dioxideparticles on the surfaces of the zeolite particles. The time for theultrasonic dispersion treatment is desirably about 1 hour. Once thetitanium dioxide particles have been adsorbed and immobilized on thesurfaces of the zeolite particles, the titanium dioxide particles cannoteasily been separated from the surfaces of the zeolite particles inwater because of the electrostatic attractive force between the titaniumdioxide particles and the zeolite particles.

The above titanium dioxide composite catalyst may be synthesized inhexavalent chromium-containing water to be treated. However,synthesizing the titanium dioxide composite catalyst in pure water ornearly-pure water in advance yields more reproducible results. In orderto immobilize the titanium dioxide particles on the surfaces of thezeolite particles, it is desirable to preliminarily subject the zeoliteparticles to activation treatment with an acid aqueous solution beforemixing the zeolite particles and the titanium dioxide particles. Zeolitehas a basic skeleton composed of silica and alumina. From anotherstandpoint, it can be said that zeolite includes (SiO₄)⁴⁻ and (AlO₄)⁵⁻as basic units. The aforementioned treatment with an acid aqueoussolution dissolves only the alumina portions (Al portions) of thezeolite, and accordingly, an increased number of active sites for directadsorption of titanium dioxide can be introduced into the basic skeletonof the zeolite. The zeolite particles having such active sites canadsorb and immobilize an increased number of the titanium dioxideparticles on the surfaces thereof. However, an acid solvent weakens theelectrostatic attractive force between the titanium dioxide particlesand the zeolite particles. Therefore, the activation treatment of thezeolite particles with an acid aqueous solution is desirably performedbefore synthesis of the titanium dioxide composite catalyst.

FIG. 2 schematically shows an embodiment of a water treatment system 2of the present embodiment. The water treatment system 2 includes apre-filtration vessel 201, a slurry tank 202, a photoreactor 203, alight source 204 provided inside the photoreactor 203, a solid-liquidseparation vessel 205, and a returning part 206. The method of thepresent embodiment includes a catalyst adding step (step a1), aphotoreaction step (step b1), and a sedimentation separation step (stepc1). The method includes a re-adding step (step d1) as necessary.Hereinafter, each of the steps will be described.

<Catalyst Adding Step (Step a1)>

The most distinctive feature of the method of the present embodiment isthat the titanium dioxide composite catalyst is irradiated withexcitation light in a state where the titanium dioxide compositecatalyst is uniformly dispersed in water to be treated.

A slurry liquid containing the titanium dioxide composite catalyst issupplied from the slurry tank 202. That is, the titanium dioxidecomposite catalyst is added to a hexavalent chromium-containing aqueoussolution held in the photoreactor 203. As a result, the titanium dioxidecomposite catalyst can be dispersed in the hexavalentchromium-containing aqueous solution in the photoreactor 203. In orderto prevent precipitation of the titanium dioxide composite catalyst, itis desirable to stir gently the aqueous solution in the photoreactor203.

The concentration of the titanium dioxide composite catalyst in thewater in the photoreactor 203 is desirably 0.4 g/L or more and 16 g/L orless. When the concentration of the titanium dioxide composite catalystin the water is more than 16 g/L, the entry of the below-described UVlight into the water is significantly disturbed, and the hexavalentchromium reduction ratio is accordingly decreased. On the other hand,when the concentration of the titanium dioxide composite catalyst in thewater is less than 0.1 g/L, the amount of the titanium dioxide compositecatalyst is insufficient with respect to the amount of UV light, whichdecreases the hexavalent chromium reduction ratio.

<Photoreaction Step (Step b1)>

UV light is applied from the light source 204. Upon the irradiation withthe UV light, hexavalent chromium contained in the aqueous solution isreduced by the catalytic action of the titanium dioxide compositecatalyst. The wavelength of the light from the light source 204 is 200nm or more and 400 nm or less. The UV light from the light source 204may be monochromatic light or continuous light as long as its wavelengthis within the above range. The shorter the wavelength of the lightapplied to the titanium dioxide photocatalyst is, the higher theefficiency of generation of excited electrons is. Therefore, a shorterwavelength of the light from the light source 204 is desirable from thestandpoint of the reduction ratio of hexavalent chromium contained inthe aqueous solution. Examples of the light source that can be used inthe photoreaction step include a low pressure mercury lamp, a mediumpressure mercury lamp, a high pressure mercury lamp, an excimer lamp, axenon lamp, sunlight, black light, and an LED. Also in the present step,it is desirable to stir gently the aqueous solution held in thephotoreactor 203.

The photoreactor 203 is either a batch-type reactor or a continuous-typereactor. Examples of the batch-type reactor include a batch reactor anda batch recirculation reactor. Examples of the continuous-type reactorinclude a stirred tank reactor and a tubular reactor. When an inorganiccompound such as mud and sand which is derived from silicas is containedin the water to be treated, it is desirable to preliminarily separateand remove the inorganic compound in the pre-filtration vessel 201.

<Sedimentation Separation Step (Step c1)>

After the photoreaction step, the aqueous solution containing thedispersed titanium dioxide composite catalyst is transferred from thephotoreactor 203 to the solid-liquid separation vessel 205. Accordingly,the stirring of the aqueous solution in the step b1 is stopped. In thesolid-liquid separation vessel 205, the titanium dioxide compositecatalyst in the aqueous solution is separated from the aqueous solutionby sedimentation, and a concentrate of the titanium dioxide compositecatalyst and treated water are produced. By making use of the fact thatthe titanium dioxide composite catalyst readily sediments, the titaniumdioxide composite catalyst is separated by sedimentation in the aqueoussolution. Examples of the process for sedimentation separation include agravitational sedimentation process, a centrifugal sedimentationprocess, and a liquid cyclone process. When hexavalent chromium isreduced, trivalent chromium is produced. The solubility of trivalentchromium is lower than the solubility of hexavalent chromium. Therefore,trivalent chromium can be removed from treated water relatively easilyby being precipitated in the treated water.

In the case of separation using a gravitational sedimentation process,the aqueous solution containing the titanium dioxide composite catalystis allowed to stand in the solid-liquid separation vessel 205. Thetitanium dioxide composite catalyst naturally sediments under the actionof gravity. Therefore, a concentrate (slurry liquid) containing theconcentrated titanium dioxide composite catalyst can be obtained in thelower portion of the solid-liquid separation vessel 205, while treatedwater that is a supernatant liquid free from the titanium dioxidecomposite catalyst can be obtained in the upper portion of thesolid-liquid separation vessel 205. The time for which the aqueoussolution is allowed to stand is, for example, 10 minutes or longer. Theconcentrate (slurry liquid) of the titanium dioxide composite catalystis returned to the photoreactor 203 via the returning part 206, and canbe reused in the photoreaction step.

In the case of separation using a centrifugal sedimentation process, thetreated water containing the titanium dioxide composite catalyst iswhirled in a holeless rotating container to separate the titaniumdioxide composite catalyst. Only the titanium dioxide composite catalystis moved toward the wall of the container by a centrifugal force, and isconcentrated. Thus, the treaded water and the concentrate of thecatalyst are separated from each other. The centrifugal force acting onthe rotating container is, for example, 60 G or more.

<Re-Adding Step (Step d1)>

The concentrate of the titanium dioxide composite catalyst, which hasbeen produced in the sedimentation separation step, is returned to thephotoreactor 203 through the returning part 206. That is, the titaniumdioxide composite catalyst separated in the sedimentation separationstep is added again to the hexavalent chromium-containing aqueoussolution. In order to reliably reduce hexavalent chromium in thephotoreactor 203, the concentrate of the titanium dioxide compositecatalyst needs to be continuously added to the to-be-treated watercontinuously flowing into the photoreactor through the pre-filtrationvessel 201. As the concentrate of the titanium dioxide compositecatalyst, the concentrate produced in the solid-liquid separation vessel205 can be reused. That is, in the present embodiment, the step b1 andthe step c1 may be further performed after the step d1. The titaniumdioxide composite catalyst can be reused as long as the surface activesites of the titanium dioxide are not covered with scale orhardly-decomposable thin films derived from organic or inorganicsubstances.

<Sedimentation Separation Performance>

In the sedimentation separation step, the catalytic particles arerequired to have the capability to be separated, for example, by agravitational sedimentation process in a short time. This capability canbe evaluated, for example, by a light transmission method. The lighttransmission method is an evaluation technique in which the change overtime in light transmittance is monitored for a suspension of a catalystby continuously measuring the transmittance of laser light with whichthe suspension is irradiated. For a suspension of a catalyst that has ahigh sedimentation velocity, a significant change in transmittance isobserved within a short time since the catalyst sediments in a shorttime. By contrast, for a substance that has a low sedimentationvelocity, almost no change in transmittance is observed even after alapse of time.

To exemplify the excellent sedimentation separation performance of thetitanium dioxide composite catalyst particles used in the presentembodiment, FIG. 3 shows how natural sedimentation proceeded for thecases of the titanium dioxide composite catalyst particles, titaniumdioxide particles (P25 manufactured by Degussa AG), and zeoliteparticles (HY type). In FIG. 3, the horizontal axis represents theelapsed time from injection of a specimen liquid into a sample, and thevertical axis represents the light transmittance. The concentration ofthe titanium dioxide composite catalyst particles in a specimen liquidwas 3.6 g/L. The concentration of the titanium dioxide particles in aspecimen liquid was 0.9 g/L. The concentration of the zeolite particlesin a specimen liquid was 2.7 g/L. The concentration of the titaniumdioxide particles and the concentration of the zeolite particles wererespectively set equal to the concentrations of the titanium dioxideparticles and the zeolite particles included in the titanium dioxidecomposite catalyst particles. As the titanium dioxide composite catalystparticles, the following two types of catalysts were prepared: acatalyst A for which zeolite particles (having a silica/alumina molarratio of 30 and a Si/Al molar ratio of 15) were used that had beentreated by being immersed in a 0.1 mol/L hydrochloric acid aqueoussolution and then stirred with an ultrasonic washer for 60 minutes; anda catalyst B for which zeolite particles were used that had been treatedby being stirred with an ultrasonic washer for 60 minutes withoutimmersion in a hydrochloric acid aqueous solution.

In FIG. 3, the numeral 301 denotes the result for the catalyst A, thenumeral 302 denotes the result for the catalyst B, the numeral 303denotes the result for the titanium dioxide particles alone, and thenumeral 304 denotes the result for the zeolite particles alone. Theprogress of the sedimentation of the titanium dioxide composite catalystwas confirmed from the increase over time in transmittance. Forcomparison of sedimentation performance, the amount of change intransmittance during the sedimentation time of 30 minutes was calculatedfor each specimen. The transmittance change in the titanium dioxideparticle specimen was 0%, and the transmittance change in the zeoliteparticle specimen was 0.026%. That is, there was almost no increase intransmittance even after 30 minutes elapsed from injection of thespecimen liquid into the sample cell. It can be said that theseparticles hardly sediment in 30 minutes. On the other hand, thetransmittance change in the catalyst A specimen was 56%, and thetransmittance change in the catalyst B specimen was 36%. This indicatesthat, regardless of whether or not the zeolite particles are subjectedto the acid treatment, both the catalyst A and the catalyst B can beseparated by sedimentation from the aqueous solution in a practical timeof about 30 minutes to an extent sufficient to allow water discharge(transmittance change of 20% or more).

The proportion (sedimentation amount) of the catalyst particles thatsedimented in 30 minutes after injection of the specimen liquid into thesample was calculated from the transmittance change. In the catalyst Aspecimen, 92% of the particles sedimented. In the catalyst B specimen,86% of the particles sedimented. That is, both the catalyst A and thecatalyst B can be separated by sedimentation in a practical time ofabout 30 minutes. By contrast, only 1.6% of the particles sedimented inthe zeolite particle specimen, and the particles did not sedimented atall in the titanium dioxide particle specimen.

When a spherical particle sediments freely in a fluid under the actionof gravity, the sedimentation velocity of the spherical particle isproportional to the difference in specific gravity between the particleand water, and to the square value of the particle diameter. In thetitanium dioxide composite catalyst of the present embodiment, thezeolite particles have a micrometer-order particle diameter (of 1 μm to10 μm, for example), and the titanium dioxide particles whose specificgravity differs from water by an amount that is 100 or more times largerthan the difference in specific gravity between the zeolite particle andwater are densely immobilized on the outer surfaces of the zeoliteparticles. Therefore, the particle diameter of the titanium dioxidecomposite catalyst is about three orders of magnitude larger than thatof a nanometer-order titanium dioxide particle, and the difference inspecific gravity between the titanium dioxide composite catalyst andwater is about two orders of magnitude larger than the difference inspecific gravity between the zeolite particle and water. These two factsare thought of as reasons why the titanium dioxide composite catalystexhibits a sedimentation velocity that is considerably higher than thoseof the zeolite particles and the titanium dioxide particles.

The zeolite particle used in the present embodiment is a porousinorganic compound having a basic skeleton composed of silica andalumina. From another standpoint, it can be said that the zeoliteparticle used in the present embodiment is a porous inorganic compoundthat includes (SiO₄)⁴⁻ and (AlO₄)⁵⁻ as basic units. The sedimentationperformance of the catalyst particles, which is key to the sedimentationseparation step of the present embodiment, is influenced by the ratiobetween silica and alumina which compose the zeolite. Here, thesilica/alumina molar ratio between silica and alumina which compose thezeolite is twice the ratio between (SiO₄)⁴⁻ and (AlO₄)⁵⁻ in the zeolite,that is, the Si/Al molar ratio. The sedimentation performance of thetitanium dioxide composite catalyst in the sedimentation separation stepof the present embodiment is influenced by the ratio between silica andalumina which compose the zeolite. Table 1 shows the transmittancechange during the sedimentation time of 30 minutes and the proportion(sedimentation amount) of the sedimented catalyst particles for titaniumdioxide composite catalysts synthesized using zeolite particles havingdifferent silica/alumina molar ratios.

TABLE 1 Silica/alumina 5 10 30 60 770 molar ratio Si/Al molar ratio 2.55 15 30 385 Transmittance 1.6 20 36 46 61 change [%] Sedimentation 43.680.0 85.9 88.9 93.2 amount [%]

As shown in the table, when the zeolite particles have a silica/aluminamolar ratio of 10 or more (a Si/Al molar ratio of 5 or more), thetitanium dioxide composite catalyst can be sedimented in a practicaltime of about 30 minutes to an extent sufficient to allow waterdischarge (transmittance change of 20% or more). The titanium dioxideparticles can be stably immobilized only when zeolite particles having asilica/alumina ratio of 10 or more are used as a support material. Thereason is that the below-described adhesion is more likely to occurbetween the titanium dioxide particles and the zeolite. Therefore, thetitanium dioxide particles can be used in water for a long period oftime without desorption from the zeolite particles. The crystal systemof the zeolite particles serving as a support material is notparticularly limited. For example, zeolite particles of a common type,such as faujasite-type particles and MFI-type particles, can be used.

In the titanium dioxide composite catalyst of the present embodiment,strong adhesions derived from electrostatic attractive force existbetween the titanium dioxide particles and the zeolite particles.Therefore, the titanium dioxide composite catalyst of the presentembodiment has high durability, and many of the titanium dioxideparticles remain immobilized on the zeolite particles even after, forexample, a lapse of several months. On the other hand, for example, whenan inorganic material such as silica particles and alumina particles, oran inorganic porous body such as brick and concrete is used as a supportmaterial, no strong adhesion occurs between the support material and thetitanium dioxide particles. In such a case, when the catalyst is stirredtogether with the aqueous solution in a photoreactor, many of thetitanium dioxide particles are disadvantageously desorbed from thesupport material in a short time.

In the present embodiment, particles of a nanometer-order size that havean anatase crystal system and a photocatalytic function, such as P25manufactured by Degussa AG, Germany, can be used as the titanium dioxideparticles immobilized on the zeolite particles. Particularly when thetitanium dioxide particles have an average particle diameter in therange of 1 nm to 100 nm, a suitable titanium dioxide composite catalystcan be formed. The average particle diameter is defined as an averagevalue of the long diameter and the short diameter of the titaniumdioxide particle. When the average particle diameter of the titaniumdioxide particle is less than 1 nm, the catalyst activity is decreaseddue to quantum size effect. In addition, when the average particlediameter of the titanium dioxide particle is more than 100 nm, thegravity acting on the titanium dioxide particle is larger than the forceacting between the titanium dioxide particle and the zeolite particle.For this reason, immobilization of the titanium dioxide particle on thezeolite particle is unstable, and the titanium dioxide particle caneasily desorb from the zeolite particle. Accordingly, thereproducibility of the titanium dioxide composite catalyst isdeteriorated. The particle diameter distribution of the titanium dioxideparticles of the titanium dioxide composite catalyst of the presentembodiment was measured using a TEM (transmission electron microscope)image. As a result, it was confirmed that the particle diameters of thetitanium dioxide particles of the present embodiment were distributed inthe range of 25.8±24.6 nm, and were within the aforementioned limits.The error is a standard deviation at 99% confidence limit.

In the method of the present embodiment, electrons generated on thesurface of the titanium dioxide under irradiation with UV light serve toreduce hexavalent chromium. When hexavalent chromium is reduced, thevalence is decreased one by one due to electrons generated byphotochemical reaction of titanium dioxide. That is, hexavalent chromiumis reduced first into pentavalent form, then into tetravalent form, andfinally into trivalent form.

<Second Embodiment>

A method of a second embodiment will be described with reference to FIG.4. The second embodiment is the same as the first embodiment, except forthe matters particularly described below.

FIG. 4 conceptually shows a water treatment system for implementing themethod of the present embodiment. The water treatment system 4 of thepresent embodiment includes a pre-filtration vessel 401, a slurry tank402, a photoreactor 403, a light source 404 provided inside thephotoreactor 403, a solid-liquid separation vessel 405, a filtrationmembrane element 406 provided inside the solid-liquid separation vessel405, and a returning part 407. The method of the present embodimentincludes a catalyst adding step (step a2), a photoreaction step (stepb2), and a sedimentation separation step (step c2). The method includesa re-adding step (step d2) as necessary, and further includes afiltration step (step e2). Hereinafter, each of the steps will bedescribed.

<Catalyst Adding Step (Step a2)>

The catalyst adding step (step a2) of the present embodiment isperformed in the same manner as the catalyst adding step (step a1) ofthe first embodiment. Therefore, a detailed description thereof isomitted.

<Photoreaction Step (Step b2)>

The photoreaction step (step b2) of the present embodiment is performedin the same manner as the photoreaction step (step b1) of the firstembodiment. Therefore, a detailed description thereof is omitted.

<Sedimentation Separation Step (Step c2)>

The aqueous solution treated in the step b2 is fed to the solid-liquidseparation vessel 405, and is allowed to stand. Accordingly, thestirring of the aqueous solution in the step b2 is stopped. The titaniumdioxide composite catalyst sediments in the aqueous solution beingallowed to stand, and a layer 410 of the precipitated titanium dioxidecomposite catalyst is formed in the lower portion of the solid-liquidseparation vessel 405. The layer 410 of the precipitated titaniumdioxide composite catalyst is separated from a mixed liquid 408 of thewater to be treated and the titanium dioxide composite catalyst, andthus a concentrated slurry is obtained. The titanium dioxide compositecatalyst in the concentrated slurry includes the titanium dioxidecomposite catalyst originally dispersed in the aqueous solution and thecatalyst particles that have not passed through the filtration membraneelement 406 in the filtration step (step e2) described below. Theproportion of the catalyst recovered in the form of the concentratedslurry is, for example, 99.99% or more. The performance evaluation andtheoretical consideration for the sedimentation of the titanium dioxidecomposite catalyst of the present embodiment are the same as describedfor the first embodiment.

<Filtration Step (Step e2)>

Microfiltration using the filtration membrane element 406 is performedsimultaneously with the sedimentation of the catalyst particles in thestep c2, so as to produce treated water 413 from the mixed liquid 408 ofthe water to be treated and the catalyst particles. The concentration ofthe titanium dioxide composite catalyst remaining in the treated waterproduced is, for example, 10 ppm or less.

FIG. 5 is a perspective view schematically showing the structure of thefiltration membrane element 406. The filtration membrane element 406includes a plate-shaped frame 501 and sheets of filter paper 502 made ofsynthetic resin and attached fixedly to both faces of the frame 501.Filtration is performed by drawing water through a filtered waterextraction port 503 using a pump (not shown). The water to be filteredpasses through the filter paper 502 made of synthetic resin, enters theinside of the frame 501, and is discharged as the treated water 413through the filtered water extraction port 503.

As shown in FIG. 4, the filtration membrane element 406 is arrangedparallel to a sedimentation direction 409 in which the titanium dioxidecomposite catalyst is sedimented. A layer of deposited catalystparticles, which is called a cake layer, is usually formed on thesurface of the filtration membrane element 406 along with the progressof the filtration. This cake layer peels off from the surface of thefiltration membrane element 406 due to its own weight, and sediments tothe lower portion of the solid-liquid separation vessel 405. Theperformance evaluation in terms of microfiltration for the titaniumdioxide composite catalyst used in the present embodiment will bedescribed later.

As described above, when hexavalent chromium is reduced, trivalentchromium is produced. The solubility of trivalent chromium is lower thanthe solubility of hexavalent chromium. For this reason, trivalentchromium is more readily precipitated in the treated water. Therefore,trivalent chromium can be removed using the filtration membrane element406.

<Re-Adding Step (Step f2)>

The re-adding step (step f2) of the second embodiment is performed inthe same manner as the re-adding step (step d1) of the first embodiment.The concentrated slurry of the titanium dioxide composite catalystseparated in the step c2 is returned to the photoreactor 403 through thereturning part 407. That is, the titanium dioxide composite catalystseparated in the sedimentation separation step is added again to thehexavalent chromium-containing aqueous solution. The titanium dioxidecomposite catalyst particles can be repeatedly reused as long as thesurface active sites of the titanium dioxide are not covered with scaleor hardly-decomposable thin films derived from organic or inorganicsubstances. That is, in the present embodiment, the step b2, the step c2and the step e2 may be further performed after the step f2.

The performance of the titanium dioxide composite catalyst of thepresent embodiment in terms of microfiltration will be described. Inorder to separate the titanium dioxide composite catalyst bymicrofiltration, the particle diameter of the titanium dioxide compositecatalyst needs to be sufficiently larger than the pore diameter of thefiltration membrane element. A larger difference between the particlediameter of the titanium dioxide composite catalyst and the porediameter of the filtration membrane element allows the titanium dioxidecomposite catalyst to be separated in a shorter time with a lowerprobability of clogging of the filtration membrane element. FIG. 6 showsthe results of particle size distribution measurements performed ontitanium dioxide particles and on the titanium dioxide compositecatalyst of the present embodiment. The numeral 601 denotes the resultof the particle size distribution measurement on the titanium dioxideparticles, and the numeral 602 denotes the result of the particle sizedistribution measurement on the titanium dioxide composite catalyst ofthe present embodiment. The average particle diameter of the titaniumdioxide composite catalyst was 5.5 μm. The average pore diameter of theresin filtration membrane element used for the microfiltration in theabove filtration step (step e2) was 0.42 μm. The average particlediameter of the titanium dioxide particles was 0.2 μm.

EXAMPLES Example 1

First, a titanium dioxide composite catalyst was produced by theprocedure described below. The zeolite particles used were HY-typezeolite particles (faujasite-type particles manufactured by Zeolyst)having an average particle diameter of 5.0 μm and a silica/alumina molarratio of 30 (Si/Al molar ratio of 15). The zeolite particles wereimmersed in a 0.1 mol/L hydrochloric acid aqueous solution, and stirredin an ultrasonic washer for 60 minutes. Subsequently, only the zeoliteparticles were separated and recovered from the water by suctionfiltration. The resultant powder was sufficiently rinsed with waterthree times to wash off the acid, and was then dried. An amount of 0.9g/L of titanium dioxide particles (P25 manufactured by Degussa AG) and2.7 g/L of the HY-type zeolite particles treated with the hydrochloricacid aqueous solution were added to pure water prepared by an ultrapurewater production apparatus. This solution was subjected to ultrasonictreatment using an ultrasonic generator for 1 hour. Thereafter, thesolution was stirred with a magnetic stirrer at a number of revolutionsof 300 rpm for 60 minutes to obtain a slurry liquid containing atitanium dioxide composite catalyst. A transmission electron microscope(TEM) image of the thus-fabricated titanium dioxide composite catalystis shown in FIG. 7C. For comparison, a TEM image of the titanium dioxideparticles alone is shown in FIG. 7A, and a TEM image of the zeoliteparticles alone is shown in FIG. 7B. It can be understood that, in thetitanium dioxide composite catalyst 703, the titanium dioxide particles701 are immobilized directly on the zeolite particle 702 without themediation of a thin film.

The treatment system shown in FIG. 2 was constructed using the slurryliquid of the titanium dioxide composite catalyst fabricated asdescribed above. The slurry of the titanium dioxide composite catalystwas loaded into the slurry tank 202, and was supplied to thephotoreactor 203. An aqueous solution having dissolved therein Kr₂Cr₂O₇,which is a hexavalent chromium compound, was used as the water to betreated. The hexavalent chromium compound was dissolved in pure water atan aqueous solution concentration of 1000 μg/L, and the resultantsolution was then introduced into the photoreactor 203. The slurryliquid of the titanium dioxide composite catalyst was supplied to thephotoreactor so that the concentration of the catalyst was 0.4 g/L.Thereafter, UV light irradiation was performed in conjunction withstirring at 200 rpm. The light source 204 was composed of a combinationof a xenon light source (MAX 302 manufactured by Asahi Spectra Co.,Ltd.) and a band-pass filter. The light had a wavelength λ of 350 nm, abandwidth of about 10 nm, and an intensity of 1 mW/cm².

The solution having been subjected to the light irradiation wascollected, and the concentrations of substances in the solution werequantitatively analyzed by HPLC/ICP•MS. The hexavalent chromiumreduction ratio achieved by irradiation for 8 minutes was 78.0%. Thus,it was confirmed that the titanium dioxide composite catalyst iseffective for reduction of hexavalent chromium, that is, detoxificationof hexavalent chromium.

The aqueous solution having undergone the photoreaction step andcontaining the suspended titanium dioxide composite catalyst wasintroduced to the solid-liquid separation vessel 205, and evaluation ofgravitational sedimentation was performed by a light transmissionmethod. A HeNe laser (632.8 nm, 3 mW, nonpolarized) was used as thelight source. A fiber coupler including an objective lens was used,laser light was introduced into the fiber, and thus the solid-liquidseparation vessel 205 containing the suspension was irradiated with thelight. The light having transmitted through the solid-liquid separationvessel 205 was introduced again into the fiber, and finally, alight-receiving surface of a photodiode (C10439-03 manufactured byHamamatsu Photonics K.K.) was irradiated with the light to measure thetransmittance. The transmittance change after the titanium dioxidecomposite catalyst was sedimented for 30 minutes was 30±5%. This valueof the transmittance change is beyond 20% which is a baseline fordetermining that the treated water is allowed to be discharged. Thus, itwas confirmed that solid-liquid separation of the titanium dioxidecomposite catalyst from an aqueous solution in which the titaniumdioxide composite catalyst is suspended can be achieved in a short timeby sedimentation separation. The titanium dioxide composite catalystseparated and recovered was able to be continuously reused by beingintroduced again to the photoreactor 203 through the returning part 206.

As described above, according to Example 1, a water treatment methodexcellent in both photocatalytic activity and solid-liquid separationperformance was achieved.

Example 2

A water treatment system for treating a hexavalent chromium-containingaqueous solution was constructed by the same way as in Example 1. A 1000μg/L hexavalent chromium aqueous solution was used as the water to betreated. The amount of the titanium dioxide composite catalyst slurrysupplied was adjusted to prepare five aqueous solutions containing thetitanium dioxide composite catalyst at different concentrations, and thehexavalent chromium reduction ratio and the sedimentation performancewere evaluated for each solution. The concentration of the titaniumdioxide composite catalyst was set to 0.04 g/L for a solution A1, 0.4g/L for a solution B1, 3.6 g/L for a solution C1, 16 g/L for a solutionD1, and 40 g/L for a solution E1. UV light irradiation was performedunder the same conditions as in Example 1, and the hexavalent chromiumreduction ratio achieved after 8 minutes was evaluated. Furthermore, asin Example 1, the aqueous solution having undergone the photoreactionstep and containing the suspended titanium dioxide composite catalystwas introduced to the solid-liquid separation vessel 205, and the lighttransmittance change after 30 minutes was evaluated as the sedimentationperformance. The hexavalent chromium reduction ratios in the solutionshaving catalyst concentrations of 0.4 g/L, 3.6 g/L, and 16 g/L were78.0%, 77.8%, and 74.3%, respectively, which revealed that highreduction ratios are achieved in these solutions. However, thehexavalent chromium reduction ratios in the solutions having catalystconcentrations of 0.04 g/L and 40 g/L were 11.2% and 13.0%,respectively, and were smaller than ⅕ of the reduction ratio in thesolution having a catalyst concentration of 3.6 g/L. That is, in thetreatment method of the present example, the suitable concentration ofthe titanium dioxide composite catalyst was in the range of 0.4 g/L to16 g/L.

Comparative Examples

For comparison, a catalyst composed of quarts beads and titanium dioxideparticles immobilized by a binder on the quarts beads was fabricated,and was evaluated for the hexavalent chromium reduction ratio and thesedimentation performance. Amorphous silica, which is commonly used, wasused as the binder. An amount of 10 g of P25 which is titanium dioxidemanufactured by Degussa AG, 8.7 g of TEOS (tetraethoxysilane) as silicaalkoxide, 20 g of ethanol, and 50 g of a hydrochloric acid aqueoussolution with a concentration of 1 mol/L, were mixed in a beaker, andthen the mixture was immediately cooled in an ice bath while beingstirred with a magnetic stirrer for 30 minutes. Evaluation was performedusing this mixed liquid within 30 minutes from the start of stirring.Quartz beads having a particle diameter of 5 μm were immersed in thismixed liquid, and the quartz beads were coated with photocatalystlayers. The quartz beads were taken out from the mixed liquid byfiltration, dried in a draft chamber for about 1 hour, and then driedfurther in an oven at 80° C. Through these steps, a photocatalystcomposed of quartz beads and titanium dioxide particles immobilized onthe quartz beads by TEOS-derived amorphous silica was obtained. It wasconfirmed by SEM observation that the titanium dioxide particles wereimmobilized on the surfaces of the quartz beads by the TEOS-derivedamorphous silica. A slurry of the photocatalyst was supplied to thephotoreactor similarly to the above, and a comparative solution 1 wasthus prepared. The concentration of the titanium dioxide particles inthe comparative solution 1 was equal to the concentration in thesolution C1 (3.6 g/L).

In addition, a solution in which only nanometer-order titanium dioxideparticles were dispersed was also tested as a comparative solution 2.P25 manufactured by Degussa AG was used as the titanium dioxideparticles, and the concentration of the titanium dioxide particles wasset equal to the titanium dioxide particle concentration in the solutionC1 (3.6 g/L). The hexavalent chromium reduction ratio and thesedimentation performance were evaluated using the same hexavalentchromium aqueous solution and the same conditions as employed in Example2.

Table 2 shows the hexavalent chromium reduction ratio and thesedimentation performance in each of the titanium dioxide compositecatalyst solutions having different concentrations and the comparativesolutions.

TABLE 2 Titanium dioxide composite catalyst solution ComparativeComparative (concentration) solution 1 solution 2 A1 B1 C1 D1 E1 (binder(titanium dioxide (0.04 g/L) (0.4 g/L) (3.6 g/L) (16 g/L) (40 g/L)process) particles) Cr(VI) reduction 11.2 78.0 77.8 74.3 13.0 9.8 83.0ratio [%] Sedimentation 33% 32% 36% 38% 35% 35% 0% performance (Changein light transmittance)

In the comparative solution 1 for which a binder was used, thesedimentation performance of the catalyst was at a similar level to thatin the composite catalyst solutions A1 to E1 of Example 2. However, thehexavalent chromium reduction ratio was very low, and specifically was9.8% which was smaller than ⅛ of that in the solution C1. In thecomparative solution 2 which was a dispersion liquid of titanium dioxideparticles, the hexavalent chromium reduction ratio was 83.0%, whichmeans that the highest decomposing performance was exhibited. However,the sedimentation performance of the catalyst was 0%; namely, thecatalyst was not separated by sedimentation at all. On the other hand,for the solutions B1 to D1 used in the present example, it was confirmedthat both the hexavalent chromium reduction performance and thesedimentation performance were so high that practical water treatmentcan be achieved.

Example 3

The water treatment system shown in FIG. 4 was constructed using aconcentrated slurry liquid of a titanium dioxide composite catalystfabricated in the same manner as in Example 1. The concentrated slurryof the titanium dioxide composite catalyst was loaded into the slurrytank 402, and was supplied to the photoreactor 403. An aqueous solutionhaving dissolved therein Kr₂Cr₂O₇, which is a hexavalent chromiumcompound, was used as the water to be treated. The hexavalent chromiumcompound was dissolved at an aqueous solution concentration of 1000μg/L, and the resultant solution was then introduced into thephotoreactor 403. The concentrated slurry liquid of the catalystparticles was supplied to the photoreactor so that the concentration ofthe catalyst was 3.6 g/L. Thereafter, UV light irradiation was performedin conjunction with stirring at 200 rpm. The light source 404 wascomposed of a combination of a xenon light source (MAX 302 manufacturedby Asahi Spectra Co., Ltd.) and a band-pass filter. The light had awavelength λ of 350 nm, a bandwidth of about 10 nm, and an intensity of1 mW/cm².

The solution having been subjected to the light irradiation wascollected, and the concentration of hexavalent chromium in the solutionwas quantitatively analyzed by HPLC/ICP•MS (manufactured by Agilent).The hexavalent chromium reduction ratio achieved by irradiation for 8minutes was 77.8%, which revealed that most of the hexavalent chromiumwas reduced by the photocatalyst. Thus, it was confirmed that thetitanium dioxide composite catalyst is effective for reduction, i.e.,detoxification of hexavalent chromium which is actually observed inwater environments such as groundwater.

The suspension solution having undergone the photoreaction step wasintroduced to the solid-liquid separation vessel 405. Then, evaluationof gravitational sedimentation was performed by a light transmissionmethod, and evaluation of separation by microfiltration was performed. AHeNe laser (632.8 nm, 3 mW, nonpolarized) was used as the light sourcefor the light transmission method. A fiber coupler including anobjective lens was used, laser light was introduced into the fiber, andthus the solid-liquid separation vessel containing the suspension wasirradiated with the light. The light having transmitted through thesolid-liquid separation vessel 405 was introduced again into the fiber,and finally, a light-receiving surface of a photodiode (C10439-03manufactured by Hamamatsu Photonics K.K.) was irradiated with the lightto measure the transmittance. A sedimentation amount was calculated fromthe transmittance. The sedimentation amount in 30 minutes was 85±5%.This value was beyond 80% which is a baseline for solid-liquidseparation of catalyst particles. Thus, it was confirmed thatsolid-liquid separation in the suspension was achieved in a short timeby sedimentation separation.

Sheets of filter paper made of resin and having an average pore diameterof 0.42 μm were used in a filtration membrane element for themicrofiltration. The output flow rate of the treated water in themicrofiltration was monitored. The output flow rate was about 100mL/min, and was stable for 24 hours. The concentration of the catalystremaining in the treated water was 10 ppm or less. By contrast, in thecase of titanium dioxide particles alone, the flow rate which wasinitially 100 mL/min was decreased to 1.2 mL/min in 8 hours, andclogging of the filtration membrane element was caused.

As described above, Example 3 verified that a water treatment methodexcellent in both photocatalytic activity and solid-liquid separationperformance was achieved using the titanium dioxide composite catalyst.

Example 4

A water treatment system for treating a hexavalent chromium-containingaqueous solution was constructed by the same way as in Example 3. A 1000μg/L hexavalent chromium aqueous solution was used as the water to betreated. The amount of the titanium dioxide compositecatalyst-concentrated slurry supplied was adjusted to prepare fiveaqueous solutions containing the titanium dioxide composite catalyst atdifferent concentrations, and the hexavalent chromium reduction ratio,the sedimentation amount, and the extraction flow rate of the treatedwater were evaluated for each solution. The concentration of thetitanium dioxide composite catalyst was set to 0.04 g/L for a solutionA2, 0.4 g/L for a solution B2, 3.6 g/L for a solution C2, 16 g/L for asolution D2, and 40 g/L for a solution E2. UV light irradiation wasperformed under the same conditions as in Example 3, and the hexavalentchromium reduction ratio achieved by 8-minute irradiation was evaluated.Furthermore, as in Example 1, a suspension having undergonephotodecomposition treatment was introduced to the solid-liquidseparation vessel, and the sedimentation amount after 30 minutes and theextraction flow rate of the treated water were evaluated. The hexavalentchromium reduction ratios in the solutions containing the titaniumdioxide composite catalyst at concentrations of 0.4 g/L, 3.6 g/L, and 16g/L were 78.0%, 77.8%, and 74.3%, respectively, which revealed that highreduction performance is exhibited in these solutions. However, thehexavalent chromium reduction ratios in the solutions having catalystconcentrations of 0.04 g/L and 40 g/L were 11.2% and 13.0%,respectively, and were smaller than ⅕ of the hexavalent chromiumreduction ratio in the solution having a catalyst concentration of 3.6g/L. That is, in the treatment method of the present example, thesuitable concentration of the catalyst in the solution was 0.4 g/L ormore and 16 g/L or less.

Comparative Examples

A photocatalyst composed of quarts beads and titanium dioxide particlesimmobilized on the quarts beads by TEOS-derived amorphous silica wasobtained in the same manner as for the above comparative solution 1. Itwas confirmed by SEM observation that the titanium dioxide particleswere immobilized on the surfaces of the quartz beads by the TEOS-derivedamorphous silica. As in Example 4, a concentrated slurry of thephotocatalyst was supplied to the photoreactor to prepare a comparativesolution 3. The titanium dioxide particle concentration in thecomparative solution 3 was equal to the concentration in the solution C2(3.6 g/L).

In addition, a solution in which only nanometer-order titanium dioxideparticles were dispersed as in the comparative solution 2 was tested asa comparative solution 4 in the same manner as in Example 4. P25manufactured by Degussa AG was used as the titanium dioxide particles,and the concentration of the titanium dioxide particles was set equal tothe titanium dioxide particle concentration in the solution C1 (3.6g/L). The hexavalent chromium reduction ratio, the sedimentation amount,and the extraction flow rate of the treated water were evaluated usingthe same hexavalent chromium aqueous solution and the same conditions asemployed in Example 4.

Table 3 shows the results for each of the catalyst particle solutionshaving different concentrations and the comparative solutions. In thecomparative solution 3 for which a binder was used, the sedimentationamount and the extraction flow rate were at similar levels to those ofthe catalyst particle solutions A2 to E2 of Example 4; however, thehexavalent chromium reduction ratio was very low, and specifically was9.8% which was smaller than ⅕of that in the solution C2. In thecomparative solution 4 which was a dispersion liquid of titanium dioxideparticles, the hexavalent chromium reduction ratio achieved was 83.0%which was the highest reduction ratio. However, the sedimentation amountwas 0%; namely, the catalyst was not separated by sedimentation at all.Moreover, the extraction flow rate was 0 mL/min; namely, the treatedwater was not able to be separated. On the other hand, for the solutionsB2 to D2 used in the present example, it was confirmed that all of thehexavalent chromium reduction ratio, the sedimentation amount, and theextraction flow rate were at such good levels that practical watertreatment can be achieved.

TABLE 3 Titanium dioxide composite catalyst solution ComparativeComparative (concentration) solution 3 solution 4 A2 B2 C2 D2 E2 (binder(titanium dioxide (0.04 g/L) (0.4 g/L) (3.6 g/L) (16 g/L) (40 g/L)process) particles) Cr(VI) reduction 11.2 78.0 77.8 74.3 13.0 9.8 83.0ratio [%] Sedimentation 81.0 82.6 85.9 88.5 89.3 89 0 amount [%]Extraction flow 110 100 105 110 100 90 0 rate [ml/minute]

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this specification are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

The present disclosure relates to detoxification of hexavalent chromiumcontained in drinking water, discharged water, continental rivers,lakes, etc., and can provide a method and system that can treat watercontinuously in a practical time. The method according to the presentdisclosure can be used for household clean water systems and publicclean water systems. Furthermore, the method can be used as one of thesteps of effluent treatment in factories or of sewage treatment process.

What is claimed is:
 1. A method for treating a hexavalentchromium-containing aqueous solution, comprising: a step (a) of addingcatalyst particles to the aqueous solution; a step (b) of reducinghexavalent chromium by irradiating the aqueous solution with ultravioletlight having a wavelength in a range between 200 nanometers and 400nanometers, both inclusive, while stirring the catalyst particles in theaqueous solution; and a step (c) of stopping the stirring in the step(b) and separating the catalyst particles from the aqueous solution bysedimentation, wherein each catalyst particle is composed only of atitanium dioxide particle having an average particle diameter in therange of 1 nm to 100 nm and a zeolite particle having a particlediameter of 1 μm to 10 μm, the titanium dioxide particle is adsorbed onan outer surface of the zeolite particle, the zeolite particle includessilica and alumina, and the catalyst particles are contained in theaqueous solution at a concentration in a range between 0.4 grams/literand 16 grams/liter, both inclusive, and a change in laser lighttransmittance of the aqueous solution from before sedimentation to aftera sedimentation duration of 30 minutes in the step (c) is 20% or more.2. The method according to claim 1, comprising a step (d) of addingagain the catalyst particles separated by sedimentation in the step (c)to the aqueous solution after the step (c), wherein the step (b) and thestep (c) are performed again after the step (d).
 3. The method accordingto claim 1, wherein the catalyst particles are separated bysedimentation in a solid-liquid separation vessel including a filtrationmembrane element in the step (c), the method further comprises a step(e) of producing treated water from the aqueous solution using thefiltration membrane element, and the filtration membrane element used inthe step (e) is composed of a plate-shaped frame and sheets of filterpaper made of resin and attached to both faces of the frame, and isarranged parallel to a direction in which the catalyst particles aresedimented.
 4. The method according to claim 3, comprising a step (f) ofadding again the catalyst particles separated by sedimentation in thestep (c) to the aqueous solution after the step (c), wherein the step(b), the step (c) and the step (e) are performed again after the step(f).
 5. The method according to claim 1, wherein the zeolite particle isa zeolite particle subjected to a process in which an alumina portion isdissolved by treatment with an acid aqueous solution to introduce anactive site for direct adsorption of the titanium dioxide particle, andthen the acid aqueous solution adhered to the surface of the zeoliteparticle is removed by washing with water.
 6. The method according toclaim 1, wherein: the step (b) is performed in a photo reactor, thecatalyst particles are transferred from the photo reactor to asolid-liquid separation vessel in the step (c), and the catalystparticles are separated by sedimentation in a solid-liquid separationvessel in the step (c).